Clarifiers to separate solids and liquids are extensively used in various industries, including the water treatment, pulp, paper, and sugar industries. A typical clarifier comprises a cylindrical tank with a sloped base, with or without a raking mechanism for removing sludge. The clarified liquid is typically withdrawn through overflow launders on the periphery of the cylindrical tank.
When manufacturing raw sugar, for example, an important preliminary step is to “clarify” the sugar juice. A general disadvantage of existing clarifier designs is that they generally allow large turbulent eddies inside the tanks, which causes inefficient use of the settling area, prolonged residence time, and entrainment of mud in the clarified liquid. These problems are exacerbated by recirculation and flotation promoted by entrained air. A degassing step is often used to remove entrained air from the sugar juice. If degassing is not performed properly, the quality of the clarified liquid will be poor.
In the production of cane sugar juice, a “flash tank” is often employed for degassing, by heating the juice slightly above the boiling point, removing air from the juice and also from entrapped particles, and then delivering the degassed juice to a sedimentation tank at a constant temperature.
Two types of “flash tanks” are commonly used. The “Type A” flash tank is a vessel that runs largely empty, to provide sufficient volume in the tank for the complete flashing of juice. The diameter is selected so that upward vapor velocities are low enough that entrainment of juice droplets does not occur. If the tank does not have an adequately sized outlet, air in the exit pipe can be re-entrained into the flashed juice.
The “Type B” flash tank runs with more liquid inside the tank during operation. The higher liquid level reduces air entrainment, but a Type B tank occupies more space than a Type A flash tank of comparable capacity. The tank has a diameter large enough that the downward liquid velocity is sufficiently slow to permit even very fine air bubbles to escape upwards against the flow of liquid. A vertical feed pipe injects the juice downwards onto a center splash plate. The liquid level is maintained just below the splash plate. The flash tank is positioned preferably adjacent to and at the same elevation as the clarifier so that a constant liquid level is maintained in the flash tank, and so that air is not re-entrained into the juice. If the liquid levels differ substantially, then air can be re-entrained into the juice in the connecting pipe. The Type B flash tank is not clean-draining, and it requires periodic cleaning. See generally P. Rein, Cane Sugar Engineering, Bartens, Berlin, 1st Ed. (2007), Sec. 9.6, pp. 213-216.
International patent application no. WO 2010/091461 A1 discloses an apparatus for a high pressure evaporator, a flash tank with a cylindrical body. The device contains a vapor outlet at the upper end that allows steam to be vented, and a slurry inlet/outlet port at the lower end.
U.S. Pat. No. 4,986,837 discloses a liquid degassing apparatus with one or more porous, hydrophobic tubes located inside a vacuum chamber. Each tube has an atomizer near its inlet. As liquid to be degassed enters the tube, it is atomized and gas in the liquid passes from the atomized liquid, through the porous tube, and exits the vacuum chamber. The remaining degassed liquid exits the tube through the downstream end.
U.S. Pat. No. 6,793,814 discloses an apparatus having a tank for settling particles out of a fluid, and having a conical chamber with a conical auger at the bottom of the tank. This device contains a tangential inlet that creates a fluid circulation, which exerts a centrifugal force in order to increase separation of heavy solid particles from relatively light fluid. An alternative embodiment provides for the collection of gas from the fluid by a degassing system located above the settling chamber. Free gas in the fluid will, when subjected to centrifugal forces, tend to collect at the center and move to the top of the tank along with the substantially clean fluid. The degassing system provides a controlled space from which gas can be removed.
Flashing of sugar cane juice is an important step to achieve a good clarification. However, its importance is often overlooked, and as a result the juice is often not properly de-gassed; air can be re-entrained; and there can be excessive levels of bagacillo (fine bagasse particles) in clear juice.